1. Field of the Invention
The invention relates to a controller for an engine and, more particularly, to a technology for controlling an engine utilizing a target engine rotational speed.
2. Description of the Related Art
In a known existing engine, an output power is determined on the basis of a throttle opening degree. In general, a throttle opening degree is in one-to-one correspondence with an accelerator operation amount. However, when the throttle opening degree is always in one-to-one correspondence with the accelerator operation amount, if the behavior of a vehicle, for example, becomes unstable, it is difficult to control a driving force, and the like, of the vehicle irrespective of driver's intention. Then, to make it possible to control an output power independently of the accelerator operation amount, some vehicles are equipped with an electronic throttle valve that is actuated by an actuator. In a vehicle equipped with the electronic throttle valve, it is possible to set a target engine torque on the basis of a behavior of the vehicle, and the like, in addition to, the accelerator operation amount and control an engine so that an actual engine torque becomes the target engine torque.
Japanese Patent Application Publication No. 2007-132203 (JP-A-2007-132203) describes a controller that controls devices of an internal combustion engine on the basis of a set target torque. The controller described in JP-A-2007-132203 includes an estimation unit that estimates a torque the internal combustion engine generates; a difference calculation unit that calculates a difference between the estimated torque calculated by the estimation unit and the target torque; a control amount calculation unit that calculates a torque control amount that compensates for response delay on the basis of the difference calculated by the difference calculation unit; and a control unit that generates command values to the devices on the basis of the torque control amount calculated by the control amount calculation unit to control the devices. The estimation unit estimates a torque using a model formula that is formed to include the response delay of the internal combustion engine. The control amount calculation unit adds a value, which is calculated using a difference calculated by the difference calculation unit and a coefficient, to the target torque to obtain the torque control amount. The coefficient is changed on the basis of the rotational speed and intake air amount of the internal combustion engine.
According to the controller described in JP-A-2007-132203, in order to achieve the target torque, the torque control amount for controlling the devices of the internal combustion engine is calculated on the basis of the difference between the estimated torque and the target torque and is compensated for response delay. Thus, the above controller compensates for response delay of the internal combustion engine, so it is possible to eliminate the response delay to improve the response of control.
Incidentally, to control the engine so as to achieve the target engine torque, it needs an engine rotational speed corresponding to the target engine torque. For example, in order to set an exhaust gas recirculation (EGR) amount, and the like, to achieve the target engine torque, it is necessary to calculate a load on the basis of an actual intake air amount of the engine and a maximum air amount the engine can take in. Then, in order to calculate the maximum air amount that the engine can take in, an engine rotational speed is necessary. However, an actual engine rotational speed corresponds to the actual engine torque, and the actual engine torque is achieved behind the target engine torque. Thus, the actual engine rotational speed detected at the time when the target engine torque is set differs from the engine rotational speed at the time when the target engine torque is achieved. For this reason, when the engine is controlled using the actual engine rotational speed together with the target engine torque, control accuracy of the engine may deteriorate. However, in the controller described in JP-A-2007-132203, the torque control amount for controlling the devices is calculated using an actual rotational speed of the internal combustion engine. Thus, there is still room for improvement in control accuracy of the engine.